Add a watchlist function to the website

[ghost]

Just to easy create a list of content that a user like to monitor by clicking a button on a mod/texture/game page. Best with the ability to notify the user when content on the watchlist was modified / updated.

[rubenwardy]

This is similar to the thread watching feature

• Add new link table, like subscribers/watches for threads
• Add UI to watch/unwatch package
• Update notification sending to also send to watchers